Download Jovian planet

Astronomy From Å to ZZ
— Howard L. Cohen
A Brief Column for the Beginning Stargazer Introducing a New Astronomical Term Each Month
A
stronomy is rich with terminology. This
column will help beginning stargazers ease into
the world of astronomy by briefly introducing a new
but basic astronomical term (word, acronym or
abbreviation) each month. This list, which began
January 1999 with the letter a, is alphabetical but uses
successive letters for each month’s entry. (We will
return to the letter a after twenty-six months.)
(The February 2001 column ended with z so we are
now in the second cycle of twenty-six terms. See the
February 2001 issue for a cumulative index.)
Note: For many people, December means Christmas
and Santa Claus. December’s word is Jovian and it is
most appropriate for this time of year. What is the
connection between Santa and Jovian? Read on!
Word of the Month for December 2001
Jovian plane t Any of the four giant planets (Jupiter,
Saturn, Uranus and Neptune) that lie beyond the
asteroid belt 1 in a region known as the Outer Solar
System.
The Jovian planets are also known as the gas giants
since they are largely composed of hydrogen and
helium. They do not have “solid surfaces” like Earth.
Instead, their gaseous atmospheres thicken and gradually change to liquid or even a “metallic liquid” under
extreme pressure (atoms are stripped of their electrons). Small cores of rock and possibly ice may exist
at their centers.
Jovian planets have many other characteristics that
help distinguish them from the smaller, rocky Terrestrial (or Earthlike) planets. The Jovian planets have
large diameters (four to eleven times Earth) and large
masses (15 to 318 times Earth) but low mean (average) densities—densities similar to water (from 0.7 to
1.8 times water). Jovian atmospheres are thick but
composed of mostly light gases. In contrast, Terrestrial planets have little or no atmospheres. Those rocky
planets that do have some atmospheric gases (Venus,
Earth and Mars), have thin envelopes of gases composed of heavier gases such as carbon dioxide, nitrogen or oxygen.
All Jovian planets rotate (spin) more rapidly than
Earth or the other Terrestrial planets (from about 10
to 17 hours). The gas giants are also very oblate
(equatorial diameters greater than polar diameters).
Therefore, they appear “flattened” at the poles. Although Saturn has conspicuous rings, all Jovian
planets also have ring systems whereas the Terrestrial
planets have none. In addition, all Jovian planets have
many satellites (from 8 to 30 but others are likely to be
found) in contrast to the rocky planets that have few
or none (Mercury, Venus and Moon2).
Thus, astronomers now realize that Jovian planets
have attributes that make them physically more like the
Sun than Earth.3
Jovian planets are named for the largest member of the
group—Jupiter. The word “Jovian” means relating to
or resembling Jupiter. In fact, the word “jovial”
derives from Jupiter. The Roman god Jupiter was
regarded as a source of “happiness” so jovial means
happy or jolly. Note also that Jupiter is a “big planet.”
In fact, Jupiter is the largest planet in the Solar System. Santa Claus is big too. Thus, Santa (who is big)
must also be jolly or “jovial” in nature!
(So, the next time you see Santa, think Jupiter. And
the next time you see Jupiter, think happy.)
'
References. J. Mitton 1991, Concise Dictionary of Astronomy (Oxford Univ. Press); I. Ridpath 1997, A Dictionary
of Astronomy (Oxford Univ. Press); M. Wagman, Sky &
Telescope, April 1991, pp. 380–381.
2
The Moon falls into the Terrestrial category of planets.
3
1
The asteroid belt is a region found between Mars and
Jupiter where most of the asteroid orbits lie. This belt helps
divide the solar system into inner and outer regions.
Only a few basic solar characteristics are not Jovian. This
includes a small oblateness. This, in turn, results from a
very slow rotation period. (The only planets that spin
slower are Mercury and Venus.)